The aim of the project is to extract and identify high value chemicals with potential applications as pharmaceutical drugs or precursors from heather (Calluna vulgaris) and bracken (Pteridium aquilinum). Supercritical carbon dioxide extracts of heather were analysed for their triterpenoid content. Several triterpenoids were identified in heather supercritical carbon dioxide extracts and many of them were already reported to have potential valuable biological activities. Seasonal variation of total triterpenoid content in heather was also evaluated. The result indicated that summer flowering heather contains the highest concentration of total triterpenoid (15400 ug/g dry plant) and spring heather picked in March exhibited the second highest total triterpenoid content (11200 ug/g dry plant). Optimal harvest and extraction time of heather is also identified based on its triterpenoid seasonal variation trend. In this study, supercritical carbon dioxide extraction of bracken was achieved for the first time. Naturally occurring pterosin B and pterosin F were identified in bracken supercritical CO2 extract and the seasonal variation in pterosin content and other major compounds in bracken were also evaluated. Bracken crozier was found to contain the highest content of pterosins, and the pterosin content reduced dramatically when the frond grew towards its maturity. Geographical difference was also found to cause significant variation in the level of pterosins. It was found that Welsh bracken contains much less pterosin B and pterosin F compared to the Yorkshire sample. This distinct difference may be due to different pH value of the soil in Kilburn (Yorkshire) and Wales. Pterosins were recently proved to have strong anti-diabetic and anti-obesity activities. Although pterosin B and F were not identified to have certain anti-diabetic or anti-obesity activities in previous research, these pterosins, especially pterosin F, have high possibility to be used as precursors to other more effective pterosins because of its active chlorinated side chain.